One aspect of the immune response to viral infection is the generation of cytotoxic T lymphocytes (CTL) which are capable of specifically recognizing and lysing virus-infected cells. While it is known that the target for CTL recognition is specified by virus encoded molecules in association with antigens encoded by the major histocompatibility complex (MHC), the nature of this association and the determinants recognized by CTL remain to be identified. The aim of this project is to identify those viral antigens on infected cells that serve as target recognition structures for virus-specific CTL and determine their association with MHC antigens. Using cloned viral genes, cell lines will be constructed that express specific viral proteins. These cell lines will be tested for their susceptibility to lysis by virus-specific CTL. The use of cloned viral genes allows specific alterations in the structure of the viral antigen to be made by site-specific mutagenesis of the viral gene. The alteration of specific regions of the viral antigen will allow the identification of those viral epitopes involved in CTL recognition. Cloned MHC genes and hybrid MHC genes constructed by exon shuffling will also be used to construct cell lines expressing specific viral proteins in association with specific MHC antigens. This will allow the study of the association of viral and MHC antigens necessary for CTL recognition. The approach proposed here should prove to be a powerful way in which the recognition of virus-infected cells by virus-specific CTL can be studied. The identification of those epitopes on viral antigens which stimulate the generation of CTL and serve as CTL recognition antigens is essential for the manufacture of more effective anti-viral vaccines and for the successful use of CTL in immunotherapeutic protocols.